The present invention pertains to a type of device commonly referred to as a "motion compensator" or "tensioner." In general, such motion compensators are employed between two relatively movable bodies or structures and compensate for the relative motion therebetween by maintaining the tension in the connecting line or lines within predetermined limits. In general, each such motion compensator includes a piston and cylinder assembly. The cylinder may be connected to one of the two relaly movable bodies or structures, while the piston is connected to the other. A cushion of pressurized air or other compressible fluid is provided at one or the other end of the cylinder as needed to resist relative movement of the piston due to increasing tension in the line or other means connecting the two bodies. Thus, as the piston moves relative to the cylinder in response to such tension, it maintains the tension in the connecting line within predetermined limits so that the line may remain sufficiently taut but without danger of breakage. In certain applications, the advantages of maintaining a relatively constant tension revolve around the ultimate objective of controlling the effective weight and/or feed rate on a drill string.
The operation of such motion compensators is more fully explained by a number of prior U.S. patents. U.S. Pat. No. 3,804,183 discloses a basic type of motion compensator and illustrates its use in interconnecting a drill string with its supporting drilling ship. U.S. Pat. No. 3,908,963 illustrates the use of such motion compensators in connecting a drilling vessel or the like with a subsurface well. U.S. Pat. No. 3,865,066 illustrates the use of such compensators in maintaining proper tension of mooring lines which connect ships to offshore platforms or the like. U.S. Pat. Nos. 3,030,893 and 3,040,667 illustrate the use of similar structures in conjunction with hydraulic pumps. U.S. Pat. Nos. 3,314,657, 4,072,122, and 3,877,680 disclose further details and/or variations of the motion compensators per se.
In some of the prior devices, such as that shown in U.S. Pat. No. 3,804,183, cited above, a relatively simple chain and sprocket arrangement is used for interconnecting the two relative movable bodies or structures via the motion compensator. However, this simple arrangement has a disadvantage in that it requires that the high pressure fluid which is used to resist increases in tension must be disposed in the rod end of the cylinder, i.e. between the piston and that end of the cylinder through which the piston rod protrudes. This in turn requires the provision of expensive, high pressure, sliding seals between the piston rod and cylinder at that end.
Other prior patents, such as U.S. Pat. No. 3,314,657, also cited above, disclose compensators arranged such that the high pressure fluid cushion is disposed in the fluid end of the cylinder, i.e. on the opposite side of the piston from the piston rod. U.S. Pat. No. 3,908,963 specifically mentions U.S. Pat. No. 3,314,657 as illustrating one type of motion compensator which can be used in connecting a vessel or the like to a subsurface well. However, devices such as illustrated in U.S. Pat. No. 3,314,657, while eliminating the need for a high pressure fluid on the rod end of the cylinder, still have some disadvantages, at least with respect to certain types of usages. Like the simpler devices described above, the device of U.S. Pat. No. 3,314,657 employs chains or other flexible members reeved about sheaves, sprockets, or the like. This makes the motion compensator relatively expensive, not only due to the use of movable parts such as the sheaves or sprockets, but also due to the need to provide the chains with sufficient strength without adversely affecting their flexibility. In addition, the type of structure shown in U.S. Pat. No. 3,314,657 ultimately employs a single common chain or set of parallel chains for connecting to both of the two relatively movable bodies or structures. Also, this type of device, while able to accommodate a relatively large degree of travel between the two bodies or structures connected thereby, necessarily sacrifices the amount of force which it can apply or withstand.